This invention relates to wrapping machinery for use in wrapping objects with thermoplastic film, such as polyvinyl chloride wrapping film, and, more particularly, to machinery for dispensing and severing sheets from a roll of such film.
It is known in the art to provide a wrapping apparatus in which film is withdrawn from a roll of film and then placed over a hot wire cut off for use in severing the film during the process of wrapping articles therein. Such apparatus is frequently employed in supermarkets for wrapping meats, vegetables, and the like prior to placing the wrapped items on display counters. In some attempts in the prior art, the hot wire was maintained at a temperature on the order of 700.degree. F. in order to effect satisfactory severance of the film. However, operating at such high temperatures caused the film to melt in such a manner as to generate obnoxious fumes. This lead to improvements in the art to achieve a severing operation while reducing the fumes. One attempt in the prior art is found in the U.S. Pat. No. 3,754,489 to H. Carver. This patent proposes that the film be severed by means of a rectangular cutoff element which is heated to a temperature in the range of 325.degree. F. to 350.degree. F., which is at or slightly above the melting point of the plastic film. The rectangular cutoff element is maintained at a substantially constant temperature along the line of cutoff during the severing operation by means of a substantial mass which provides a heat sink to maintain a constant temperature during severing.
Another approach in the industry has taken the form of the U.S. Pat. No. 4,014,229, to W. Lynch, assigned to the same assignee as the invention herein and the disclosure of which is herein incorporated by reference. In Lynch, supra, the cutoff element takes the form of a metal tube having an internal electrical heating element which is electrically insulated from the tube and which heats the cutoff element to a temperature on the order of 240.degree. F. to 260.degree. F. which is below the melting point of the plastic film. The cutoff element preferably has a corner edge extending transversely of the path of movement of the film and this corner edge serves to concentrate stresses applied to the film along the line of severance to assist in severing the film from the film roll. The cutoff element is associated with a thermostat, which senses the temperature of the cutoff element and switches the electrical power to the heating element contained therein in dependence upon the sensed temperature.
A problem noted in the operation of such prior art film dispensing and severing apparatus is that, after the film has been severed, the leading edge of the remaining film may lie against the cutoff element. If there is a substantial period of time between successive severing operations, this leading edge will tend to melt and leave a plastic coated deposit on the cutoff element. This deposit detracts from the appearance of the cutoff element and provides a dull corner edge on the cutoff element. More importantly, however, this plastic coated deposit will add a layer of insulation on the cutoff element, decreasing the thermal efficiency of the cutoff element. Therefore, it is desirable that after each severing operation, the leading edge of the remaining film should back away from the cutoff element a sufficient distance so that it will not tend to melt and leave deposits on the cutoff element and will not provide fumes. To some extent this can be achieved in the apparatus disclosed in Lynch by applying a braking force to the roll of plastic film so that the film is under tension as it is being withdrawn from the roll. After the severing operation, the leading edge will then back up toward the roll. However, depending upon the amount of braking force applied to the roll, the film will rapidly snap back toward and gather about the roll resulting in the need for rethreading the leading edge of the film. This results in down time, which should be avoided in the operation of such a wrapping apparatus.
The present invention proposes that a pair of pinch rollers be located intermediate the roll of film and the cutoff element for guiding the passage of the film to the cutoff element. Further, the present invention proposes that a one-way clutch be associated with at least one of the pinch rollers to prevent the film, after severance, from rapidly backing up and gathering about the film roll. Such a construction, then, permits a severing operation with a heated cutoff element without creating fumes and without the film tending to melt and provide plastic deposits on the cutoff element.
It is, however, known in the art to provide a film wrapping machine having pinch rollers located intermediate the roll of film and a cutoff element. Such is illustrated in the patent to F. Treiber, U.S. Pat. No. 4,501,106. Specifically, Treiber, supra, discloses pinch rollers including a roller 370 which is described in the patent at column 12, lines 31-35 as including a one-way clutch to allow rotation only in a clockwise direction, as shown in FIG. 12 of the patent. It should be noted, however, that whereas Treiber, supra, discloses pinch rollers having a one-way clutch, they are not employed in a wrapping apparatus using an electrically heated cutoff element as in the case of Lynch, supra. Instead, Treiber, supra, employs a mechanical cutoff knife, as is shown in FIG. 6 of the patent. Consequently, Treiber is not concerned with the need for ensuring that the leading edge of the film does not lie against the cutoff element which would result in obnoxious fumes or in which the film would tend to melt and leave deposits on the cutoff element as discussed hereinabove.